Collection systems for collecting flat objects, e.g., sheets or envelopes made of paper or flat cards made of a paperboard or plastic material, and forming stacks of individual objects or subgroups of such objects are well known from the prior art. For this purpose, for example, assembling lines can be used, which lines comprise a conveyor unit with a plurality of collecting points disposed one behind the other, at which collecting points the objects are deposited in stacks and removed by the conveyor unit. The objects are stored in feeder stations which are disposed above and along the conveyor unit in such a manner that the objects can be deposited at the defined collecting points. This type of assembling line is known, for example, from U.S. Pat. No. 4,177,979 B. However, collecting flat objects in the form of stacks by means of such an assembling line is time-consuming and not very flexible. In order to deposit the objects at a predefined collecting point along the assembling line, the function of the feeder stations has to be synchronized, for example, by means of a complex mechanical coupling, such as is described in U.S. Pat. No. 4,177,979, to ensure that the feeder stations can deposit the objects stored therein correctly positioned at the predefined collecting points along the assembling line. The prior-art assembling lines allow the objects to be stacked only in a stacking direction from the bottom to the top, i.e., using such an assembling line, the objects can be collected only in an ascending direction in the form of a stack of objects and be deposited at the collecting points along the assembling line.
Also known from the prior art are stationary collecting stations which are able to receive serially supplied objects which are collected in the form of a stack and subsequently removed. Such stationary collecting stations again allow the objects supplied to be collected in a stack only in an ascending order in the sequence of their arrival at the collecting station. With this system, the formation of subgroups is possible.
To allow flat objects, such as sheets, to be stacked in the descending order as well, i.e., from the top to the bottom, collection systems with movable switching elements are known from the prior art, which switching elements are able to divert objects serially transported along a transport line into different directions so that the objects diverted from the original transport direction can be deposited either onto the top surface of a sub-stack already present in a collection tray or can be pushed under the sub-stack. This allows the objects to be collected in a stack both in an ascending as well as in a descending order. Collection systems of this type are known, for example, from DE 93 01 072 U1 and EP 0 655 983 B1. However, these collection systems with a switchable diverting device have a complex design structure and are susceptible to faults when in operation. Difficulties can arise when a group or subgroup comprising a plurality of single sheets is pushed under an already deposited sub-stack, for example, when the sheets have punched holes or notches. Because of the weight of the sub-stack and high friction between the upper surface of the objects or the subgroup of objects to be pushed under the sub-stack and the lower surface of the already present sub-stack, it is often difficult to slide the objects or subgroups of objects under the sub-stack without kinking the objects. In this type of collection systems, difficulties arise when stacks are to be formed in the descending order, in particular, with folded sheets, especially when these sheets have different folding lengths. When sliding a subgroup of folded sheets or a single folded sheet under a previously stacked sub-stack, there is a risk that the folded sheets will become entangled with one another, which can ultimately lead to a sheet jam in the collection system or to damage to the sheets.
Disclosed in U.S. Pat. No. 5,435,534 is a system for and a method of folding and collecting paper sheets, said paper sheets first being individually fed to a dual collection system by means of a conveyor unit. The dual collection system comprises two collection trays disposed one on top of the other, each of which trays defines a collection plane. To deposit a paper sheet supplied by the conveyor unit into one or the other collection tray, a switchable control element is disposed between the conveyor unit and the collection trays, which switchable control element routes the arriving paper sheets either into the upper or into the lower collection tray. Disposed downstream of the dual collection system is a folding device, by means of which the sheets collected in the form of stacks in the dual collection system can be folded. Since the folding device is able to fold only stacks of sheets with a limited height, an entire group of sheets, which, for example, is to be inserted into an envelope, is first divided into two subgroups by depositing them in the two collection trays of the dual collection system, with a first subgroup being deposited in the upper collection tray and a second subgroup in the lower collection tray of the dual collection system, each subgroup being deposited in the form of a stack. After dividing the group of sheets into the first and the second subgroup, first the second subgroup is removed from the lower collection tray and fed to the folding device disposed downstream in which first the second subgroup is folded and subsequently forwarded to an additional collection unit having a single collection tray. Subsequently, the first subgroup is removed from the upper collection tray of the dual connecting system and supplied to the folding device in which also the first subgroup is folded and ultimately deposited onto the folded stack of the second subgroup in the additional collection unit. The stacks of sheets deposited in the collection trays of the dual collection system are removed one after the other from the respective collection tray and fed to the folding device downstream thereof. This process is time-consuming and slows down the stacking procedure. As a result, the throughput of the collection system is reduced.
In collection systems comprising a plurality of collection trays disposed one on top of the other, an additional problem is that, while being deposited into one of the collection trays, the objects which are frequently supplied at very high speeds in a range of several meters per second bounce back because of the high kinetic energy and as a result thereof can block the entrance of the tray or even fall out of the collection tray.